Why does it keep on giving me this error in c

Question

float fahrenheit;
float celsius;
float formulaf = (fahrenheit - 32.0) * 5/9;
char str[20];
float formulac = celsius * 9/5 + 32.0;
printf("Choose either fahrenheit or celsius; ");
scanf("%*s", &str);
if (strcmp(str, "fahrenheit") ==0)
{
    printf("Enter the temperature in fahrenheit: ");
    scanf("%f", &fahrenheit);
    printf("%.2f fahrenheit is %.2f celsius", fahrenheit, formulaf);
}
else
{
    printf("Enter the temperature in celsius: ");
    scanf("%f", &celsius);
    printf("%.2f celsius is %.2f fahrenheit", celsius, formulac);
}

I'm coding this in xcode, I have created the headers and everything, this is the main part where I am stuck, it gives an error in the following code and it gives breakpoints in some other lines, please help me solve this

scanf("%*s", &s); -------> data argument not used by format string

what does this mean?

You need to study how integers and floating point variables work. `5/9` doesn't make any sense in C, for example. — Lundin, Jun 24 '15 at 06:50

score 2 · Answer 1 · answered Jun 24 '15 at 06:30

2

If you read e.g. this scanf reference you will see that the "*" modifier in the format string means the scanned string will not be used.

Also, when scanning for strings, you should not use the address-of operator, as strings already are pointers.

answered Jun 24 '15 at 06:30

Some programmer dude

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Spikatrix · Answer 2 · 2015-06-24T06:36:51.447

1

%*s instructs scanf to scan and discard the sting scanned. The compiler is complaining because the second argument of that scanf(&s) is not used.

You probably wanted to use %s or %19s(tells scanf to scan a maximum of 19 characters + 1 for the NUL-terminator) which prevent buffer overflows.

Also change the second argument of scanf to s. This is done because %s expects a char* while &s is of type char(*)[20]. s gets converted into &s[0], a char*, exactly what %s expects.

So the scanf

scanf("%*s", &str);

should be

scanf("%s", str);

for the reasons explained above.

BTW, Your code exhibits Undefined Behavior because when you use

float formulaf = (fahrenheit - 32.0) * 5/9;

and

float formulac = celsius * 9/5 + 32.0;

farenheit and celsius are uninitialized. Move it after they get initialized,i.e, move it after

scanf("%f", &fahrenheit);

and

scanf("%f", &celsius);

respectively.

edited Jun 24 '15 at 06:36

answered Jun 24 '15 at 06:29

Spikatrix

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And can you please explain why should i not use &str in my code ? – user5032369 Jun 24 '15 at 07:19
@user5032369 , It might have worked for your friend. But undefined behavior is undefined. It might work, might not, or do something weird. It might stop working on adding a simple `printf` somewhere etc etc. Read the third paragraph of my answer to know why you need to use `str` instead of `&str`. Alternatively, browse through similar questions on StackOverflow like [this](http://stackoverflow.com/questions/8921617/c-why-no-for-strings-in-scanf-function) – Spikatrix Jun 24 '15 at 07:26

Why does it keep on giving me this error in c

2 Answers2